arduino-adafruit shild - readDigitalPin func

hello, im working on a project Including arduino-adafruit motor shild and a stepper motor.

I attached a hall efect sensor to the arduino (at pin 2) and a magnet to the motor, now i want that the motor will stop moving when the magnet is aboce the senor.

I checked it through matlab and the arduino gets feedback from the sensor just fine. my problem is that when im Writing it on a loop, it wont let me read the pin more than once.

here is the code

a = arduino('com4', 'uno', 'Libraries', 'Adafruit\MotorShieldV2');
switch_check=readDigitalPin(a,2);
sm=Rotate();
for i=1:200
  if switch_check==1 %depends on the switch (normally open or normally closed
      move(sm,1);
      switch_check=readDigitalPin(a,2);%problematic, need to check why!!!
  else
      disp('the pin is 0')
  end
end

and this is what i get after one time the loop is running

"Instrument object OBJ is an invalid object."

i can really use your help thank you

[Geoff Hayes]

sagie - which function call is generating this error: the move or the readDigitalPin?